The dogs have the agreeable trait of taking half their wounds and then fleeing from him (towards the enemy since he shambles less than they march). When you blow the brayhorn in round 2 they rally and he shambles up and finishes them off.

Beastmen are more valuable for their ability to wrap around him and restrict his movement/visibility. While he's invincible from beyond 12" there remains the threat of enemies moving forward and gunning him down with a ratling gun or some such, or charging him with some sort of long charging character killer. In addition to this, the Beastmen can often drop a spawn without needing an immediate panic check.

I also like to keep some chariots around, as they usually take one or two wounds and craft some spawn, while retaining their full charge strength.

Note that when clumping up around Morghur you need to leave everyone a path straight towards their closest foe, in case they go Unruly and move during the compulsory phase, which will totally mess up your march forward, as they go 1", bump into the guys ahead of them and then stop, fouling the guys behind them. In particular, Morghur's own path forward must be left open insofar as you want him to go.